1. Field of the Invention
The invention relates to block copolymers comprising at least one polyalkadiene block and at least one (aromatic vinyl-co-acrylonitrile) block, and the use thereof as additive for polymers.
Mixtures of polymers can in principle give a wide variety of mechanical properties by adjusting the type and the amount of polymers in mixtures. In particular, mixtures are prepared for the purpose of modifying the impact strength, particularly at lower temperatures, the dimensional stability and the processability.
However, most polymers show no proper miscibility with other polymers, so that no homogeneous product is formed, but a polymer product with many (macroscopic) domains formed by the individual polymers. Such products have no good mechanical properties, in particular not as regards ductility.
Moreover, mixtures of different polymer fractions often occur when reprocessing polymers. In fact, a waste fraction will seldom consist 100% of the same polymer. To yet obtain a product having stable mechanical properties, a homogeneous polymer product must also be realized.
Thus, mixtures of polyethylene, e.g. low density PE, and polyvinyl chloride are inherently brittle. These mixtures are interesting both from the viewpoint of modifying the (mechanical) properties of PVC and for considerations of recycling.
2. Description of the Related Art
In the prior art a lot of research has been done on block copolymers to be used as compatibilizing agent for mixtures of polymers, in particular homopolymers, which per se do not mix together; and as modifier for specific mechanical properties of polymers. Compatibilizing agents reduce the interfacial tension between the immiscible components, so that the polymer mixture can form a dispersion with much smaller domains. These dispersions often have improved ductility.
In spite of the large number of research studies from the prior art, it is still not completely clear which criteria a copolymer must satisfy so as to act as an effective compatibilizing agent. Besides simple diblock copolymers which, on the basis of theoretical predictions, should have the most effective compatibilizing activity, different more complex copolymers have been proposed. Thus, in the prior art random copolymers, graft copolymers, tapered diblock copolymers and low molecular triblock copolymers have been proposed which have an effectiveness higher than that of the simple diblock copolymers.
For the above-mentioned example of polyethylene and polyvinyl chloride, inter alia graft copolymers of PVC or polymethyl methacrylate grafts on polyethylene polymers have been proposed. Thus, an article by Boutevin et al. in Polymer Bulletin 14 (1985) 25-30 describes that the mechanical properties of a PE/PVC mixture can be optimized by adding at least 5 wt. % of a graft copolymer of polyethylene with methyl methacrylate. Besides, the optimization in properties is qualitatively and quantitatively hardly spectacular.
An article by Hajian et al. in Eur. Polym. J. 20 (1984), 135-138 describes the effect of specific copolymers on mixtures of PE and PVC and of PE and polystyrene. Acceptable results are only obtained if the copolymers are used in amounts above 10%.
Block copolymers of, e.g., polybutadiene (PB) and polystyrene (PS) are used to compatibilize, inter alia, polyethylene/polystyrene and polypropylene/polystyrene mixtures. These block copolymers are usually prepared by anionic techniques. Anionic techniques have a number of evident disadvantages. They require strict polymerization conditions, e.g. very pure monomers, an absolute moisture- and oxygen-free atmosphere and cryogenic temperatures. These conditions are hard to realize on an industrial scale.
Moreover, no random copolymers can be obtained by using anionic techniques.
European patent application 0 418 118 describes copolymers prepared by using iniferters.
In an article in Polym. J. 17 (1985) 97, Otsu and Kuriyama describe a process in which random and alternating copolymer sequences could be prepared by using the iniferter technique.
By the term "iniferter" are designated specific free radical initiators which simultaneously also function as chain transfer agent and as polymer radical terminator.